The Science of Global Warming

The Uncertainties of the Science of Global Warming

Although much is known about the physics of global warming there are many uncertainties in regard to the amount of warming that will occur. This is the result of the complexity of the Earth's system, the internal positive feedback processes, the lack of long term global temperature and CO ₂ records, variations in solar output and the Milankovitch cycles, the affect of aerosols and the affect of the ocean as a sink for carbon dioxide.

The Earth is a remarkably complex system that involves atmosphere, land masses, oceans and ice formations. There are so many factors that affect this massive thermodynamical system. Some of the processes are understood, but many of these processes have a high degree of uncertainty and our current modeling is insufficient to make accurate predictions. One of the primary sources of the uncertainty is the positive internal feedback processes. In a positive feedback process it is necessary to understand how each component of the global warming problem affects the other components. An example is the role played by clouds. Water vapor contributes the most to the greenhouse affect. As temperatures increase, there is more evaporation. Water vapor forms clouds. Clouds increase the albedo of the Earth by reflecting more light back into space and therefore decrease the fraction of the solar energy absorbed. This is evidenced by the fact that clouds are white, which means that they are reflecting a lot of light. By reflecting the incoming sunlight, unlike the light that reaches us from clouds, clouds could have a cooling affect on the Earth. On the other hand, clouds are a greenhouse gas and therefore cause a heating of the Earth. This heating, however, results in a melting of the Earth's ice which decreases the albedo of the Earth and results in warming. So the answer is not obvious whether cloud formation has a warming or cooling effect. This is the challenge of understanding positive internal feedback processes.

We only have an accurate record of global temperatures for the last one hundred and fifty years and CO ₂ records for the past sixty years. Prior to that time we must rely on other methods to determine the temperature and CO ₂ levels. Remarkably we can acquire these in the ice cores of the poles and Greenland that go back 650,000 years. This can be done because the ice can be dated. There are two methods of dating the ice. The first method is to read the signature caused by the regular magnetic fluctuation of the sun that has an average cycle of 11 years. The second method is only accurate for a much shorter time, but because the ice thaws and melts every year, there is a mark in the ice like the rings of a tree. The temperature can be obtained because there is an incredibly accurate correlation between temperature and the concentration of the ^{1 8}O isotope. The carbon dioxide level is obtained by analyzing the air trapped in bubbles in the ice. The limitation of this is that although we are able to get very accurate readings of temperature and carbon dioxide concentrations we only have the temperature at a few locations on the globe where there are long standing glaciers. Local temperatures are variable so this limits our ability to project and to create models from the data.

There are some significant uncertainties about the influence of the sun on the Earth. There are variations of solar output. The sun's luminosity was 70% when the Earth first formed and is increasing by 6% per billion years. The climate is very sensitive to the sun's variability. The changes in the sun's position is known as the Milankovitch cycle resulting from precession, rotation of the semi-major axis and the change of eccentricity of the Earth's orbit. The Milankovitch cycle is very periodic and it's signature is evident in the long term climate and in fact correlates strongly with the occurrence of ice ages.

Aerosols, which are small particles that remain in the atmosphere, are another source of uncertainty. They are reflective of the solar energy but their exact affect is unknown. Although burning fossil fuels result in aerosols, the majority of aerosols are the result of volcanic eruption. More research needs to be done to determine the effect of aerosols on global warming.

The ability of the oceans to absorb CO ₂ is a huge uncertainty in the global system. It has been assumed that the oceans are able to absorb large quantities of CO ₂. This appears to have been the case in the past but there are indications that the oceans are warming, although there is a time lag, and that an increase in temperature could cause the oceans to release CO ₂. This is the reverse of the expectation for climate models and would significantly increase global warming. In addition, it is clear that the oceans are being drastically affected by the absorption of CO ₂ because the pH of the oceans is dropping since CO ₂ is acidic. This decrease in pH is reducing the coral reefs ability to capture CO ₂ in the form of carbonates thus increasing the atmospheric concentrations as well. The human production of greenhouse gases is having a global impact with consequences beyond our ability to foresee. It is time for us to begin to limit the damage that we are doing to the planet and control the production of greenhouse gases before it is too late.